Slow DNS Resolution on Ubuntu Linux Server 14.05 LTS

This all started with WordPress timeouts. I was trying to activate some premium plugins, and the license activation was timing out. I started doing some digging and found they use the WordPress core library WP_http which in turn uses curl to make the request. I wrote my own code to use WP_Http and it failed in the same way with a timeout. I added a timeout parameter to the wp_remote_get() call, and it was able to complete without a timeout. I then used a IP address in place of the domain name and it worked without the need for the timeout parameter.

Shell

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<?php

$response=wp_remote_get('http://google.com');

print_r($response);

echowp_remote_retrieve_body($response);

?>

With that info in hand, I decided it must be on the server. I started doing some tests:

Looks like the resolver sends parallel requests, fails to see the IPv6 response, waits 5 sec and sends sequential requests because it thinks the nameserver is broken. By adding the options single-request, glibc makes the requests sequentially be default and does not timeout.

I found some good info and hints on this issue here: https://bbs.archlinux.org/viewtopic.php?id=75770

Lastly, to bring this whole thing full circle, the WprdPress plugins now are able to get out and communicate successfully. Woohoo!